1. Field of the Invention
The present invention relates to a power supplying apparatus which converts received DC power into AC power and supplies it to a load.
2. Description of the Related Art
A conventional type of a power supplying apparatus of this type comprises a chopper circuit, which converts DC power into pulse power through ON/OFF switching, adjusts the power level of the pulse power by changing the ON/OFF ratio (duty ratio) and smoothes the pulse power to output DC power, and an inverter circuit which is provided at the succeeding stage of the chopper circuit and converts the output of the chopper circuit into AC power by subjecting this output to ON/OFF switching at the duty ratio of 50%. Normally, the duty ratio of the chopper circuit is 50% or below, and the chopper circuit is rendered conductive during part of the period in which the inverter circuit is ON. The switching frequency of the chopper circuit is generally set to an integral multiple of the switching frequency of the inverter circuit. Provided the switching frequency of the chopper circuit is set equal to that of the inverter circuit or less than twice the latter switching frequency, when the ON duration of the chopper circuit slightly shifts, the chopper circuit will not be rendered conductive during the ON duration of the inverter circuit so that no voltage is applied to the inverter circuit from the chopper circuit. This renders the operation of the inverter circuit unstable Further, with the inverter circuit being of two-arm type, when the ON duration of the chopper circuit slightly shift, there would appear some difference in power distributed to the individual arms of the inverter circuit, which may also render the operation of the inverter circuit unstable.
As a larger transformer needs to be used in the inverter circuit for low-frequency operation, the inverter circuit should be operated at a high frequency in order to make this circuit compact. For this purpose, there is a demand to increase the switching frequency of the inverter circuit. In this case, the chopper circuit needs to be operated at a frequency which is an integral multiple of the switching frequency of the inverter circuit. The operation frequency of the chopper circuit is restricted by the switching ability of the switching elements in this circuit. Therefore, increasing the frequency of the inverter circuit is influenced and restricted by the switching frequency of the chopper circuit, so that the frequency of the inverter circuit cannot be increased so much.